The parents of Connor, Marybeth Scheidts and David Levy, turned to the technique after three attempts at intrauterine insemination. The couple finally opted for IVF, which produced 13 embryos, seven of which looked normal.
Ordinarily, one of the seven would be randomly selected for implantation. However, the couple had enrolled in a clinical trial of an embryo screening technique, which revealed that just three of the embryos carried the correct number of chromosomes. Two of those were frozen, the other was implanted.
IVF accounts for up to 5 percent of all births in developed countries, but it is very inefficient. An estimated 80 percent of embryos either don’t implant or miscarry.
“If you take a woman in her early 30s, around a quarter of her embryos will be abnormal. For a woman in her early 40s, it’s around three-quarters,” said Dagan Wells at Britain’s University of Oxford, who pioneered the new technique. The problem is that many abnormal embryos look normal under a microscope. “We need better ways of working out which embryo is the one that we should implant,” Wells said.
In the Scheidts-Levy case, he took cells from the seven embryos and extracted their DNA. He then used next-generation sequencing (NGS) to count the chromosomes in each cell.
Previously, Wells’s team had compared DNA from healthy cells, abnormal embryos and cells with chromosomal abnormalities. These formed a library with which DNA from the seven embryos could be compared.
Three were found to be normal, and one of them was implanted into Scheidts’s womb, resulting in the birth of Connor.
The hope is that by selecting only healthy embryos for implantation, more women will become pregnant and fewer will experience miscarriages. Recent clinical trials of a related technique, pre-implantation genetic screening, have suggested that the method might boost the implantation rate. The big advantage of using NGS is that multiple embryos can be screened simultaneously, significantly reducing costs.
“This isn’t going to solve the problem of reproductive aging, as a couple in their early 40s may find they have no healthy embryos from which to choose,” Wells said, but it should boost the success rate for IVF in younger women.
The technique can be tweaked to allow examination of any gene mutations, such as those that cause cystic fibrosis. “This potentially gives us the opportunity to look at multiple genes and chromosomal copies,” said Stuart Lavery of IVF Hammersmith, a fertility clinic in London. “It gives us a very powerful tool for pre-implantation genetic diagnosis.”
The fact that NGS can be done on a single cell from a human embryo raises ethical questions. Last year, researchers sequenced the entire genome of an 18-week-old fetus using fragments of fetal DNA in the mother’s blood along with DNA samples from both parents. Wells’s technique shows that this can be done before an embryo has even implanted in the uterus.
“It shows that there is the potential for getting an unprecedented amount of information about an embryo before it’s transferred to the womb,” Wells said. “We need to be very careful that this isn’t used for trivial, nonmedical reasons.”
“At some point in the not-too-distant future, prospective parents will have the technical ability to look at the genome of their embryos and select embryos based on the traits they see, whether those are disease traits, cosmetic traits, behavioral traits, or boy or a girl,” said Hank Greely of Stanford University’s Center for Law and the Biosciences.
A worldwide ban on such selection might be unworkable. “There are roughly 200 countries in the world,” he said. “If 199 ban it, that’s a great commercial opportunity for the 200th.”
— New Scientist